Kabul, Afghanistan (CNN) -- Three U.N. staff members were killed in a firefight in central Kabul on Wednesday morning, a U.N. spokesman said.

The staff members were living in a private guesthouse near where an attack took place, according to Adrian Edwards of the United Nations. It was unclear whether they were the target of the attack or were in the wrong place at the wrong time, he said.

The Afghan Interior Ministry confirmed an assault in the heart of the city, but provided no additional details. Afghan police and soldiers could be seen in the area.

Weapons fire and explosions ripped through the heart of the capital, starting about 6 a.m. local time. The fighting began as sporadic gunfire, but intensified over time, lasting more than an hour.

The attack took place in a relatively secure section of the capital, in the vicinity of a number of government buildings.

The firefight, which included machine-gun fire and rocket-propelled grenades, appeared to be concentrated in one location. A plume of black smoke could be seen rising in the area.

The fighting comes 10 days ahead of a presidential runoff election on November 7. Taliban militants have threatened to disrupt the polling.